Synthesis, micellization, and thermally-induced macroscopic micelle aggregation of poly(vinyl chloride)-g-poly(N-isopropylacrylamide) amphiphilic copolymer

Abstract

A series of poly(vinyl chloride)-g-poly(N-isopropylacrylamide) (PVC-g-PNIPAM) amphiphilic copolymers with different graft lengths and densities were synthesized via the single electron transfer-living radical polymerization (SET-LRP) of NIPAM using poly(vinyl chloride-co-allyl α-bromoisobutyrate) as macroinitiator. The living nature of SET-LRP grafting copolymerization was verified by the kinetics study and narrow molecular weight distribution of PNIPAM grafts. The chemical structure, micellisation, and thermally-induced multistep aggregation of PVC-g-PNIPAMs were investigated. PVC-g-PNIPAMs form micelles comprised of a PVC core and PNIPAM corona in water at room temperature. These micelles are thermoresponsive and show a lower critical solution temperature (LCST). The micelle size and LCST of PVC-g-PNIPAM increase with increasing the graft density and length of PNIPAM. PVC-g-PNIPAM exhibits a very unique aggregation behavior above its LCST and forms a three-dimensional macroscopic aggregate with a well-defined and tunable shape at an extremely low concentration (∼0.1 wt%). The aggregate shrinks to a more compact structure with the further increase of temperature. Higher copolymer concentration, longer graft length, and lower graft density are favorable for the macroscopic micelle aggregation of PVC-g-PNIPAMs. A self-standing and superporous PVC-g-PNIPAM material having an extremely low density of ∼0.01 g cm⁻³ and a high porosity of >99% is attained after freeze-drying the micelle aggregate.